1. Technical Field
The present invention relates to an atomic oscillator.
2. Related Art
As an oscillator having high-accuracy oscillation characteristics on a long-term basis, an atomic oscillator is known which oscillates on the basis of energy transfer of atoms of an alkali metal such as rubidium or cesium. In general, an operating principle of the atomic oscillator is classified broadly into a method using a double resonance phenomenon based on light and microwaves and a method using a quantum interference effect (CPT: coherent population trapping) based on two types of light beams having different wavelengths.
In any type of atomic oscillator, an alkali metal is sealed in a gas cell together with a buffer gas, and the gas cell is required to be heated to a predetermined temperature by a heater in order to maintain the alkali metal in a gaseous state. Here, in general, the alkali metal within the gas cell partially changes to liquid as a surplus portion without being wholly gasified. Such surplus alkali metal atoms change to liquid by separating (condensing) out to a low temperature portion of the gas cell. However, when the alkali metal atoms are present in a region through which excitation light passes, the atoms shield the excitation light, which results in a deterioration in oscillation characteristics of the atomic oscillator.
Consequently, in a gas cell disclosed in JP-A-2007-324818, a concave portion for separating an alkali metal is provided at a position away from an optical axis of excitation light. A portion apart from the concave portion of the gas cell is heated by a heater, thereby making the temperature of the concave portion lower than that of the peripheral portion thereof. Thus, a surplus portion of the alkali metal is stored in the concave portion as liquid, which preventing the surplus portion from shielding the excitation light.
However, in a case where the atomic oscillator disclosed in JP-A-2007-324818 is made small, heat generated by the heater is transmitted to the entire atomic oscillator depending on the size of the atomic oscillator. For this reason, even the temperature of the concave portion rises. As a result, the surplus portion of the alkali metal is not stored in the concave portion as liquid, and thus there is the possibility of the surplus portion shielding the excitation light. In this manner, it is difficult to partially change the temperature of the gas cell of the atomic oscillator which is reduced in size.
JP-A-2015-122597 discloses a quantum interference device which is included in an atomic oscillator. The quantum interference device disclosed in JP-A-2015-122597 includes a heating unit that transmits heat, which is supplied from the heating unit, to a gas cell and a heat radiation unit that forms a low temperature portion in the gas cell, thereby preventing a surplus portion of an alkali metal from condensing on a path through which excitation light passes. Therefore, it is possible to obtain the quantum interference device with high reliability.
In the quantum interference device disclosed in JP-A-2015-122597, heat movement (heat transmission) is performed in the heating unit connected to the gas cell or the heat radiation unit due to a close contact between the gas cell and a connection portion. However, as is well known, so-called “variations in dimensions” occur at the time of manufacturing a component. Variations also occur in a gap between the gas cell and the heating unit or a gap between the gas cell and the heat radiation unit due to the variations in dimensions. For example, when the gap becomes larger, an air layer formed in the gap serves as a heat insulating layer, which remarkably degrading the efficiency of heating or heat radiation of the gas cell. In particular, the degradation in the efficiency of heat transmission in the heating unit destabilizes the temperature of the gas cell, thereby reducing the reliability of the quantum interference device. Alternatively, in a state where components overlap each other without a gap formed therebetween and interfere with each other, a large load is applied to a glass gas cell, which leads to a concern that the gas cell may be damaged or broken.